Flat affect refers to a 'flatter' display of emotions. It usually refers to a flat facial expression or monotone voice. Seeing as the majority of people exhibit a wide variety of expressions and tone when experiencing different emotions, those who don't are prone to getting misunderstood.
Flat affect is also considered a negative 'symptom' by neurotypicals. The top search results for flat affect show pages (Healthline, VeryWell Health) that encourage 'treatment' by learning how to 'create appropriate responses' that don't naturally occur to them. VeryWell Health also explained how flat affect can negatively affect someone's social relationship, but they imply that the responsibility lies in the person to learn the 'appropriate' expressions and tone of voice—basically encouraging masking.
A more detailed information written by an autistic person: https://medium.com/@duncanvm/autism-and-flat-affect-63381dbeda9e